In recent years, sophisticated information communication is advanced particularly with progress of the computer technology. For example, in the field of automobile, quantity of information to be exchanged among electronic controllers for controlling loaded electronic instruments is increasing rapidly. Consequently, a multiplex communication system is employed in order to reduce the number of wire harnesses used for transfer of information.
In this multiplex communication system, nodes such as a control ECU or the like are connected to the common communication line for exchanging data frames among the nodes. Since the communication line is used in common, various access systems are proposed for mediation of collision between data frames on the communication line.
As the access system, those like a CSMA/CD in which each node detects signals on the communication line to mediate collision between data frames depending on the priority information included in the header of data frame are known. In these access systems, the node to transmit the data frame stops transmission of data frame upon detection of the priority which is higher than that of data frame and thereby the data frame having the highest priority acquires the right of transmission.
In the case of automobiles which require a variety of controls, many nodes are sorted to the groups of different attributes such as communication rate or the like to form a plurality of networks in order to assure efficient data communication, a data relay device is provided as a common node for a plurality of networks and communication between the nodes in different networks is conducted through the data relay devices.
A node generally executes the management of transmission timing to assure smooth transmission of data frames. For example, in the node from which the data from a sensor to detect conditions of the control objects is transmitted periodically, when many sensors are used, the data can be transmitted by dispersing the data frames within such period.
However, since the transmission timing is not adjusted between the nodes in the access system such as CSMA/CD, if the transmission timing is controlled within each node, the data frames may be transitionally centralized to the particular node and are then transmitted depending on the scale of network.
This event is easily generated in the data relay device. Therefore, it is also proposed that the priority is set for the data to a higher level so that the data frame to be relayed or relayed does not remain for a longer period in the relay device or the priority is set again in the data relay device. Under this situation, a load of communication rises at a time in the network of the data relaying (repeating) destination. Particularly, this condition may be often observed in the relaying process to the network of lower communication rate from the network of higher communication rate and capacity.
As a result, the transmission timing of data frames is excessively delayed in the node of the network in the relaying destination.
Moreover, a situation will arise, in which not only the transmission timing is delayed but also the preceding data still remains in the transmission queue when the data to be transmitted is generated, depending on the capacity of a buffer to temporarily store the data to be transmitted. In this case, the data in the transmission queue is lost when the newly generated data is over-written. For example, when the data of the same kind to be obtained in the same time series are lost in a part of the period or when the data such as a switching operation information generated abruptly is generated, an apparatus does not follow the switching operation and a switch operator understands such operation as a fault of apparatus.